Plasma arc torches are widely used in the cutting of metallic materials. A plasma arc torch generally includes an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the torch or any of a variety of contact starting methods.
One objective in the design and development of plasma arc torches is to improve the cutting speed of the torches. Several factors influence cutting speed including: cutting current; standoff height; plasma plenum pressure; and swirl strength. Another factor which influences the cutting speed is the shape and size of an exit orifice of the nozzle through which the plasma arc exits the torch.
The nozzle includes a hollow, body portion defining a cavity and a substantially solid, head portion formed integrally with the body portion. The head portion defines an exit orifice extending from the chamber. The exit orifice has an inlet and an outlet. The diameter of the cavity is typically several orders of magnitude larger than the diameter of the exit orifice. The inner profile of the cavity typically corresponds to the outer configuration of the electrode, which generally has a cylindrical or conical configuration. In prior art plasma torches, the inlet of the exit orifice is typically characterized by a sharp or square edge. In addition, the exit orifice typically has a length to diameter ratio of less than about 2.4. For example, a plasma arc torch manufactured and sold by Hypertherm, Inc. under the product name POWERMAX800 includes a nozzle having an exit orifice with a length to diameter ratio from about 1.8 to about 2.4. The POWERMAX800 torch has a maximum cutting speed of about 30 ipm (inches per minute). The nozzle in the POWERMAX800 torch has an exit orifice with a sharp edged inlet.
One consideration in designing a plasma arc torch involves minimizing the potential for "double arcing" which damages the electrode and the nozzle of the torch. Double arcing is the creation of two arcs, one between the electrode and the nozzle and a second arc between the nozzle and the workpiece. The likelihood of double arcing increases as arc constriction, which increases the temperature and voltage of the plasma arc, increases.
It is therefore an object of the present invention to provide an improved nozzle design for a plasma arc torch, which increases the maximum cutting speed of plasma arc torches without increasing the likelihood of double arcing during torch operation.